SET-IGNORE-OK

The first example above allows unused formals and locals, i.e., variables
that would normally have to be declared ignored or ignorable.
The second example disallows unused formals and locals; this is the default.
The third example allows them, but prints an appropriate warning.

One might find this event useful when one is generating function
definitions by an automated procedure, when that procedure does not
take care to make sure that all formals are actually used in the
definitions that it generates.

Note: Defun will continue to report irrelevant formals even if
:set-ignore-ok has been set to t, unless you also use
set-irrelevant-formals-ok to instruct it otherwise.